1. Field of the Invention
The present invention is directed to methods for identifying nuclear magnetic spectra from spatially selectable regions of an examination subject which is disposed in a fundamental magnetic field and varying gradient fields as well as being subjected to a sequence of RF pulses for exciting nuclear magnetic resonance metabolites in the subject.
2. Related Art
Nuclear magnetic resonance devices are known in the art wherein nuclear magnetic resonance signals are obtained from an examination subject which is disposed in a fundamental magnetic field and various gradient fields and which has been subjected to a sequence of RF pulses. The resulting nuclear magnetic resonance signals are acquired using a coil disposed in the proximity of the examination region, and spatial selection is achieved by superimposing the transmission/reception characteristic of the coil with a selective slice excitation signal obtained by the combination of a selected gradient field and a frequency-selective RF pulse.
Full topical resolution is generally not employed in nuclear magnetic spectroscopy. The demarcation of individual volume regions of an examination subject, for example, an organ, is desired. In a method known from an article by Bottomly et al. in the Journal of Magnetic Resonance, Vol. 59, 1984, pages 338-342, the desired volume is selectively excited for this purpose. The reception as well as the transmission of the nuclear magnetic resonance signals occurs by means of a surface coil applied to the examination subject. The reception characteristic and the transmission characteristic of the surface coil are thereby used in combination for the volume selection. Then, during the selective excitation, a slice is selected by connecting a magnetic field gradient. A portion of the slice can then be selected on the basis of the characteristic of the surface coil.
This known method, however, presents problems for spectroscopy of compounds having a short relaxation time T.sub.2. This is because the selective excitation necessarily lasts a relatively long time. This relatively long selective excitation time is a consequence of the fact that an RF pulse having a bandwidth corresponding to the slice thickness must be used. The free induction decay (FID) signal, which cannot be interpreted until the end of the excitation, has thus already decayed to a relatively great degree given their short T.sub.2 times. This is particularly true for in-vivo phosphorus spectroscopy.
It is known from U.S. Pat. No. 4,021,726 that spatial selection for imaging a selected slice of an examination subject can be achieved by saturation (canceling) all spins except those in the selected slice. Read-out of the desired information is accomplished using a 90 degree excitation pulse, which is applied simultaneously with a read-out gradient. This method, however, is not suitable for spectroscopy because the presence of the read-out gradient would destroy the spectral information.
It is known from U.S. Pat. No. 4,816,764, that spatial presaturation pulses may be used to selectively saturate spins adjacent to a surface coil away from an area of interest. The employed method, however, spoils the signal in one area away from the coil and therefore provides only one degree of spatial selection.
It is known from U.S. Pat. No. 4,319,90 that one method of implementing additional degree of spatial localization for spectroscopic data acquisition employs additional phase encoding of the received signal. This phase encoding is subsequently transformed into a spatial encoding with some post-processing steps (e.g. a Fourier transform). The disadvantage of this technique is that only phase encoding is used for spatial localization. For some examinations, spectroscopic signals may be obscured due to high signals adjacent to volumes of interest.
In order to further localize spectroscopic signals which are spatially encoded using the above-described method, a hybrid technique was employed which uses a series of selective pulses to delineate a selected voxal of interest. This selected voxal was further subdivided by the phase encoding method described above. The disadvantage of this technique is that it can require a relatively long period of time to select the voxal of interest to be further spatially encoded. During this time, much signal is lost due to the T.sub.1 and T.sub.2 decay processes.